In the search for hydrocarbons and in the evaluation of coal and synfuel type deposits, drilling represents an expensive commitment. A drilled hole can become worthless unless basic logs taken in such a hole can provide information vital to evaluating the potential of the surrounding area. If the lithology of the area is simple and known, analysis of the logs will give good values of the parameters needed to evaluate a reservoir and to determine reserves. However, the complex composition of mixed lithologies being encountered worldwide in oil and gas exploration and production have greatly complicated log analysis and current log analysis methods are not adequate in many areas. The need is for logs to specifically and quantitatively identify basic rock types (limestone, dolomite, sandstone, and mixtures of these), clay type and volume, and minerals that adversely affect the logs used to estimate reserves. The practice of cross plotting the basic porosity logs (sonic, density, and neutron) to identify lithology is at best semiquantitative even though used for quantitative corrections. This practice introduces error of unknown magnitude in the basic log-derived parameters of porosity, oil saturation and water saturation.
Large surface area smectite clays, such as montmorillonite, are often associated with low resistivity (high water saturation) sands that produce almost water-free oil or gas. The basic logs correctly show these formations to have water saturations in the range of 60 to 90. Clean sands that show this range of water saturation produce much water and little if any oil or gas. Water is bound to the clay surface and is not produced in one case but is freely produced in the other. The problem is that the basic logs don't show a unique difference between the two types of formations.